1. Field of the Invention
The invention relates to a film printed circuit board in which long, narrow electrodes, which are provided at a board opening, are laid over the other electrodes, these electrodes being coupled with solder, as well as a liquid crystal display in which liquid crystal cells and wiring boards are connected through a film printed circuit board.
2. Related Art
In liquid crystal displays, a plurality of data lines and gate lines from liquid crystal cells, which are composed of a pair of glass substrates and liquid crystal which is sealed between these glass substrates, are connected with an integrated circuit and on/off switching is controlled by this integrated circuit, thereby displaying images.
An integrated circuit is mounted on a TAB, which is a wiring circuit board for connecting liquid crystal cells and wiring boards. In a TAB, circuit patterns, including integrated circuits, are printed on a film base (for example, polyimide), and the electrodes on the output side of the integrated circuit are connected with the electrodes of said liquid crystal panel body. Moreover, an FPC (Flexible Printed Circuit), etc., can be considered as a wiring circuit board.
In this state, the electrodes on the input side of the integrated circuit and the electrodes of said wiring board are electrically connected by soldering.
As shown in FIG. 7, the electrodes 102 on the input side of the conventional TAB 100 are bridged in long, narrow forms parallel to each other by printing between the opposing lengths of the slit openings 104 (dimension of long side: L; dimension of short side: W) provided on the TAB 100 so that the heat of the soldering iron is sufficiently conveyed. In FIG. 8B, the input electrode 102 is shown only in the opening 104, but in reality, the extended section of the input electrode 102 is printed on the bottom surface of the TAB 100. With the compactness of a liquid crystal display and functional advancement, the precision of the pitch dimension of this input electrode 102 becomes higher. Some with the input electrodes of 0.25 mm in width, and a gap between the input electrodes 102 of about 0.25 mm have already been materialized.
A number of electrodes 108 similar to said input electrodes 102 in shape are arranged opposite the input electrodes 102 on the surface of the wiring board 106. The solder 110 has been applied on these board electrodes 108 in advance.
When the TAB 100 and the wiring board 106 are overlaid in this state, the input electrodes 102 and the basic electrodes 108 are overlaid through the solder 110. When they are heated thereafter, the solder 110 is melted, thereby allowing both the electrodes 102 and 108 to be electrically connected with each other.
However, in the conventional connection (soldering) above, the pitch between the input electrodes 102 was very narrow (0.25 mm) and the melted solder 110 flowed to the adjacent input electrodes 102 (the direction shown by the arrow F in FIG. 8) through the gap (equivalent to the thickness of the board electrode 108) between the periphery of the opening 104 and the wiring board 106 by capillary action, resulting in short-circuiting of the electrodes in some cases (See FIG. 8). Moreover, in FIG. 8A, the flow state of the solder 110 is painted over black for explanation, but this cannot be seen in reality because these portions are located between the TAB 100 and the wiring board 106.
In addition, when an external force is generated such that the TAB 100 and the wiring board 106 move relatively in the direction shown by the arrow A in FIG. 7, the shearing stress is concentrated at the joint portion of the input electrode 102 at the opening 104, as shown in FIG. 9. Breaking of wire may occur at the joint section of this input electrode 102 at the opening section 104.
It is difficult to repair these defects. Because of this, short-circuiting or wire breakage at this soldered section was the biggest factor in increasing the percent defective at companies. Therefore, the lowering of this defect percentage will improve yield and be a most important subject in reducing the loss of profit, but the measures for it have not yet been obtained.